In general, extrusion processing of aluminum alloy and the like is high in the versatility in terms of the sectional shapes and is excellent for acquiring a hollow material formed by extrusion. Thus, it is being widely employed in these days. Recently in particular, products manufactured by extrusion processing have come to be used broadly as strong members of structural materials, mechanical components, and the like. Thus, there are increasing demands for extruded members constituted with high-strength alloys, particularly with maximum strength aluminum alloys such as the so-called 7000-system, e.g., 7075, 7N01, and 7003.
As an example of a conventional extrusion die for forming a hollow material, there is known a hollow-material extrusion die constituted with the so-called a spider die in which a male die and a female die are mounted inside a die ring (see Patent Document 1, for example).
As shown in FIG. 20, a spider die 100 disclosed in Patent Document 1 is constituted by including: a male die 101 having a core (mandrel) 110 for forming an inside shape of a hollow material; and a female die 102 for forming an outside shape of the hollow material. The male die 101 is constituted by including the mandrel 110 and a male ring 112 that holds the mandrel 110. Further, the mandrel 110 is formed with a forming projected part 113 and bridge legs 111 for holding the forming projected part 113.
Further, a distal-end peripheral side surface 115b of a distal-end 115 of the bridge leg 111 forms a slope surface that expands towards the tip side of the extrusion direction. The distal-end peripheral side surface 115b is fitted with an inner peripheral surface 112a of the male ring 112.
The mandrel 110 includes, on the bottom side thereof, a part that forms the inside shape of the hollow material. In the outer periphery of the mandrel 110, the bridge legs 111 in an X-letter shape, for example, i.e., extended in four directions, towards an inner periphery slope surface 112a of the male ring 112 are provided. Further, a space surrounded by the four bridge legs 111 and the inner peripheral surface 112a of the male ring 112 is a space S for introducing a billet formed with an aluminum alloy as a material.
The male die 101 is held by the female die 102 at the extrusion direction tip side shown with an arrow A. A forming hole part 106 to which the bottom part of the mandrel 110 is inserted and which is used for forming the outside shape of the hollow material is formed in the female die 102. Further, a holding surface 116 for holding the bottom surfaces of the bridge legs 111 of the male die 101 is formed on the outer periphery side top surface of the female die 102.
As described above, each of the bridge legs 111 in the spider die 100 disclosed in Patent Document 1 is formed as the slope surface in which the distal-end periphery side surface 115b of the distal-end 115 becomes expanded towards the tip side of the extrusion direction. Thus, during the extrusion of the billet, the axial force works on each of the bridge legs 111 and the bending stress working on each of the bridge legs 111 is decreased. Thus, the flexure of each of the bridge legs 111 is suppressed, thereby providing a structure with which the holding state of the mandrel 110 during the extrusion becomes stable.    Patent Document 1: Japanese Unexamined Patent Publication Hei 7-124633
In a case where a high-strength alloy, particularly the so-called 7000-system maximum strength aluminum alloy, is used as a material for forming a hollow material and an extruded material having a plurality of hollow parts such as a material in a sectional shape having a rectangle with two vertically parallel lines or the like is formed as a member for automobile dampers, for example, to be formed with the alloy, it is difficult to increase the speed of extrusion and to improve the life of the die since the deformation resistance thereof is higher than those of other alloy types so that the extrusion processing force becomes greater and the load for the die tools becomes greater as well.
For example, the hollow material extrusion die 100 disclosed in Patent Document 1 described above is so structured that the inner periphery slope surface 112a of the male ring 112 and the distal-end periphery side surfaces 115b of the bridge legs 111 are press-fitted to generate a compression stress to the bridge legs 111 in the direction orthogonal to the extrusion direction. The pressure stress and the extrusion force applied to the top surfaces of each of the bridge legs 111 when extrusion processing is executed, i.e., the tensile force for pulling towards the extrusion direction tip side generated in the shaping extrusion part 113, are set off thereby to prevent damages of the bridge legs 111 and to prevent damages of the mandrel 110 as a result.
However, in the extrusion die 100, the distal-end parts 115 of the bridge legs 111 are sloped in the direction spreading towards the tip side of the extrusion direction. Thus, the distance L between a base end part P1 held on the holding surface 116 of the female die 102 in the distal-end part 115 of the bridge leg 111 and the intersection point between the bridge leg 111 and the shaping extrusion part 113, i.e., a working point P2 that may be broken by the tensile force, becomes larger, so that the moment is increased.
Therefore, when an extrusion force is applied to the mandrel 100, a large weight is applied to the working point P2 so that the bridge legs 111 may be broken.
In order to overcome this issue, it is considered to increase the strength of the bridge legs 111 by increasing the size of the bridge legs 111 or to reduce the moment by shortening the distance L between the base end part P1 and the working point P2.
However, when the size of the bridge 111 is increased, the introduction space S of the billet to which the billet is guided and housed becomes smaller. Thus, the set amount of the billet cannot be secured. In order to secure the set amount of the billet, it is necessary to increase the inside diameter of the male ring 112. To do so, the die becomes large-sized and the distance L is extended, so that the moment cannot be reduced as a result.
Further, when the distance L between the base end part P1 and the working point P2 is shortened, the space between the male ring 112 and each of the bridge legs 111, i.e., the introduction space of the billet S, becomes small. This causes such issues that the extrusion amount of the billet is reduced, etc., so that there is naturally a limit in shortening the distance L.
As described above, with the spider die 100 designed to overcome the issues by offsetting the compression stress and the tensile stress, there is a possibility of breaking the bridge legs 111 as well as the mandrel 110 as a result. Thus, there is also a limit in extending the life of the die.
In order to overcome the issues, it is an object of the present invention to provide an extrusion die for forming a hollow material, which is capable of performing high-speed extrusion and preventing breakage of the mandrel at the same time so as to extend the life even when extrusion-forming a billet (an extruded material) constituted with a high-strength alloy with a high extrusion processing force, particularly constituted with the so-called 7000-system maximum strength aluminum alloy.